The present invention relates to the field of turbomachines; it relates to the damping of a blade made of composite and is aimed more especially at damping turbojet engine fan blades.
Blades, particularly fan blades but also low-pressure compressor blades, made of carbon fiber composite, are produced in different ways. According to one method of manufacture, a stack of unidirectional plies or woven prepregs is formed and placed in a mold, orienting the successive plies differently, before compacting and polymerizing in an autoclave. According to another method, woven dry fiber prepregs are prepared and stitched together or alternatively, a single three-dimensional woven fiber or filament preform is created and then impregnated with resin by injection molding in a closed mold. The blade is made as a single piece comprising the root and the vane. It has various protective features to enhance its thermomechanical strength. Hence, a metallic protection is attached to the leading edge or to the entire contour of the vane comprising the leading edge, the blade tip and the trailing edge, this for example being in the form of a titanium component bonded to the entire surface of the leading edge and to a forward portion of the exterior surfaces of the extrados and of the intrados. Likewise, the exterior face of the intrados is reinforced by fitting a protective film that may be made of a synthetic material, for example polyurethane, bonded directly to the intermediate component.
The invention is aimed at this type of blade that has protection at least along the leading edge. One example of manufacture is described in patent EP 1.777.063 in the name of the present applicant.
Flutter is a phenomenon involving coupling between the aerodynamics and the elastic characteristics of the blade creating unstable situations. Flutter manifests itself asynchronously. A distinction is drawn between subsonic flutter and supersonic flutter. A fan blade is chiefly affected by subsonic flutter.
Flutter is a phenomenon that is difficult to predict because of the complexity of the coupling between the aerodynamic and mechanical responses. Further, mechanical blade damping is also generally not very well understood. Finally, in the present-day design of a blading that is being subjected to increasingly high loading, flutter is a phenomenon that needs to be taken into consideration quite especially.